Why is ExecuteResult not called? - asp.net

Question

I'm trying to implement a JSONP response in my ASP.NET MVC 5 web application. I've followed this tutorial:
http://www.ingeniumweb.com/blog/post/using-jsonp-calls-in-asp.net-mvc/1216/
The problem is that the ExecuteResult method is never called:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Script.Serialization;
namespace subscribe.ActionResults
{
public class JsonpResult : JsonResult
{
object Data = null;
public JsonpResult()
{
}
public JsonpResult(object Data)
{
this.Data = Data;
}
public override void ExecuteResult(ControllerContext ControllerContext)
{
if (ControllerContext != null)
{
HttpResponseBase Response = ControllerContext.HttpContext.Response;
HttpRequestBase Request = ControllerContext.HttpContext.Request;
string callbackfunction = Request["callback"];
if (string.IsNullOrEmpty(callbackfunction))
{
throw new Exception("Callback function name must be provided in the request!");
}
Response.ContentType = "application/x-javascript";
if (Data != null)
{
JavaScriptSerializer serializer = new JavaScriptSerializer();
Response.Write(string.Format("{0}({1});", callbackfunction, serializer.Serialize(Data)));
}
}
}
}
}
In my controller I have:
public JsonpResult Get(string emailaddress, string callback, string jsonp)
{
var slist = from u in db.subscriptions
select u;
if (!String.IsNullOrEmpty(emailaddress))
slist = slist.Where(s => (s.emailaddress == emailaddress));
if (slist.Count() > 0)
{
JsonpResult result = new JsonpResult(slist.ToList());
return result; //json list of matches
}
else
{
JsonpResult result = new JsonpResult(new List<Subscription>());
return result; //empty json list
}
}